This invention is related to the recovery of minerals by froth flotation.
Flotation is a process of treating a mixture of finely divided mineral solids, e.g., a pulverulent ore, suspended in a liquid whereby a portion of the solids is separated from other finely divided mineral solids, e.g., silica, siliceous gangue, clays and other like materials present in the ore, by introducing a gas (or providing a gas in situ) in the liquid to produce a frothy mass containing certain of the solids on the top of the liquid, and leaving suspended (unfrothed) other solid components of the ore. Flotation is based on the principle that introducing a gas into a liquid containing solid particles of different materials suspended therein causes adherence of some gas to certain suspended solids and not to others and makes the particles having the gas thus adhered thereto lighter than the liquid. Accordingly, these particles rise to the top of the liquid to form a froth.
The minerals and their associated gangue which are treated by froth flotation generally do not possess sufficient hydrophobicity or hydrophilicity to allow adequate separation. Therefore, various chemical reagents are often employed in froth flotation to create or enhance the properties necessary to allow separation. Collectors are used to enhance the hydrophobicity and thus the floatability of different mineral values. Collectors must have the ability to (1) attach to the desired mineral species to the relative exclusion of other species present; (2) maintain the attachment in the turbulence or shear associated with froth flotation; and (3) render the desired mineral species sufficiently hydrophobic to permit the required degree of separation.
A number of other chemical reagents are used in addition to collectors. Examples of types of additional reagents used include frothers, depressants, pH regulators, such as lime and soda, dispersants and various promoters and activators. Depressants are used to increase or enhance the hydrophilicity of various mineral species and thus depress their flotation. Frothers are reagents added to flotation systems to promote the creation of a semi-stable froth. Unlike both depressants and collectors, frothers need not attach or adsorb on mineral particles.
Froth flotation has been extensively practiced in the mining industry since at least the early twentieth century. A wide variety of compounds are taught to be useful as collectors, frothers and other reagents in froth flotation. For example, xanthates, simple alkylamines, alkyl sulfates, alkyl sulfonates, carboxylic acids and fatty acids are generally accepted as useful collectors. Reagents useful as frothers include lower molecular weight alcohols such as methyl isobutyl carbinol and glycol ethers. The specific additives used in a particular flotation operation are selected according to the nature of the ore, the conditions under which the flotation will take place, the mineral sought to be recovered and the other additives which are to be used in combination therewith.
While a wide variety of chemical reagents are recognized by those skilled in the art as having utility in froth flotation, it is also recognized that the effectiveness of known reagents varies greatly depending on the particular ore or ores being subjected to flotation as well as the flotation conditions. It is further recognized that selectivity or the ability to selectively float the desired species to the exclusion of undesired species is a particular problem.
Minerals and their associated ores are generally categorized as sulfides or oxides, with the latter group comprising oxygen-containing species such as carbonates, hydroxides, sulfates and silicates. Thus, the group of minerals categorized as oxides generally include any oxygen-containing mineral. While a large proportion of the minerals existing today are contained in oxide ores, the bulk of successful froth flotation systems is directed to sulfide ores. The flotation of oxide minerals is recognized as being substantially more difficult than the flotation of sulfide minerals and the effectiveness of most flotation processes in the recovery of oxide ores is limited.
A major problem associated with the recovery of both oxide and sulfide minerals is selectivity. Some of the recognized collectors such as the carboxylic acids, alkyl sulfates and alkyl sulfonates discussed above are taught to be effective collectors for oxide mineral ores. However, while the use of these collectors can result in acceptable recoveries, it is recognized that the selectivity to the desired mineral value is typically quite poor. That is, the grade or the percentage of the desired component contained in the recovered mineral is unacceptably low.
Due to the low grade of oxide mineral recovery obtained using conventional, direct flotation, the mining industry has generally turned to more complicated methods in an attempt to obtain acceptable recovery of acceptable grade minerals. Oxide ores are often subjected to a sulfidization step prior to conventional flotation in existing commercial processes. After the oxide minerals are sulfidized, they are then subjected to flotation using known sulfide collectors. Even with the sulfidization step, recoveries and grade are less than desirable. An alternate approach to the recovery of oxide ores is liquid/liquid extraction. A third approach used in the recovery of oxide ores, particularly iron oxides and phosphates, is reverse or indirect flotation. In reverse flotation, the flotation of the ore having the desired mineral values is depressed and the gangue or other contaminant is floated. In some cases, the contaminant is a mineral which may have value. A fourth approach to mineral recovery involves chemical dissolution or leaching.
None of these existing methods of flotation directed to oxide ores are without problems. Generally, known methods result in low recovery or low grade or both. The low grade of the minerals recovered is recognized as a particular problem in oxide mineral flotation. Known recovery methods have not been economically feasible and consequently, a large proportion of oxide ores simply are not processed. Thus, the need for improved selectivity in oxide mineral flotation is generally acknowledged by those skilled in the art of froth flotation.